Low Chemical Index Of Alteration Research Articles

Inorganic geochemistry of El’gygytgyn Lake sediments (northeastern Russia) as an indicator of paleoclimatic change for the last 250 kyr

The inorganic geochemistry of sediments from El’gygytgyn Lake shift in phase with interpreted paleoclimatic fluctuations seen in the record over the past 250 ka. Warm periods, when the lake was seasonally ice free and fully mixed, are characterized by increased concentrations of SiO2, CaO, Na2O, K2O, and Rb, by decreased contents of TiO2, Fe2O3, Al2O3, and MgO, and by a lower chemical index of alteration (CIA). Increased levels of SiO2 reflect increases in limnic productivity whereas many of the other elements and the CIA likely reflect increased hydrological activity coincident with an increase in coarser sand and silt content and a decrease in clay mineral content. For cold/cooler periods when perennial lake ice cover lead to a stratifed water column and anoxic bottom waters, the opposite is generally observed suggesting a decrease in hydrological activity and an increase in post-depositional chemical alteration.

Reply to: Comments on “Trace element and isotopic evidence for Archean basement in the Lonar crater impact breccia, Deccan volcanic province” by Ramananda Chakrabarti and Asish R. Basu

The ∼1.88 km diameter Lonar impact crater formed ∼570 ka ago and is an almost circular depression hosted entirely in the Poladpur suite of the ∼65 Ma old basalts of the Deccan Traps. To understand the effects of impact cratering on basaltic targets, commonly found on the surfaces of inner Solar System planetary bodies, major and trace element concentrations as well as Nd and Sr isotopic compositions were determined on a suite of selected samples composed of: basalts, a red bole sample, which is a product of basalt alteration, impact breccia, and impact glasses, either in the form of spherules (<1 mm in diameter) or non-spherical impact glasses (>1 mm and <1 cm). These data include the first highly siderophile element (HSE) concentrations for the Lonar spherules. The chemical index of alteration (CIA) values for the basalts and impact breccia (36.4–42.7) are low while the red bole sample shows a high CIA value (55.6 in the acid-leached sample), consistent with its origin by aqueous alteration of the basalts. The Lonar spherules are classified into two main groups based on their CIA values. Most spherules show low CIA values (Group 1: 34.7–40.5) overlapping with the basalts and impact breccia, while seven spherules show significantly higher CIA values (Group 2: >43.0). The Group 1 spherules are further subdivided into Groups 1a and 1b, with Group 1a spherules showing higher Ni and mostly higher Cr compared to the Group 1b spherules. Iridium and Cr concentrations of the spherules are consistent with the admixture of 1–8 wt% of a chondritic impactor to the basaltic target rocks. The impactor contribution is most prominent in the Group 1a and Group 2 spherules, which show higher Ni/Co, Ni/Cr and Cr/Co ratios compared to the target basalts. In contrast, the Group 1b spherules show major and trace element compositions that overlap with those of the impact breccia and are characterized by high EFTh (Enrichment Factor for Th defined as the Nb-normalized concentration of Th relative to that of the average basalt) as well as fractionated La/Sm(N), and higher large ion lithophile element (LILE) concentrations compared to the basalts. The relatively more radiogenic Sr and less radiogenic Nd isotopic composition of the impact breccia and non-spherical impact glasses compared to the target basalts are consistent with melting and mixing of the Precambrian basement beneath the Deccan basalt with up to 15 wt% contribution of the basement to these samples. Variations in the moderately siderophile element (MSE) concentration ratios of the impact breccia as well as all the spherules are best explained by contributions from three components – a chondritic impactor, the basaltic target rocks at Lonar and the basement underlying the Deccan basalts. The large variations in concentrations of volatile elements like Zn and Cu and correlated variations of EFCu-EFZn, EFPb-EFZn, EFK-EFZn and EFNa-EFZn, particularly in the Group 1a spherules, are best explained by evaporation-condensation effects during impact. While most spherules, irrespective of their general major and trace element composition, show a loss in volatile elements (e.g., Zn and Cu) relative to the target basalts, some spherules, mainly of Group 1, display enrichments in these elements that are interpreted to reflect the unique preservation of volatile-rich vapour condensates resulting from geochemical fractionation in a vertical direction within the vapour cloud.

Petrology and geochemistry of the Neoproterozoic Nguba and Kundelungu Groups, Katangan Supergroup, southeast Congo: Implications for provenance, paleoweathering and geotectonic setting

The Nguba and Kundelungu Groups constitute the middle and upper parts of the Neoproterozoic Katangan Supergroup, respectively, and consist of conglomerates, sandstones, mudrocks and carbonates. During deposition, the Katangan basin received sediments originating from both northern and southern sources. The Nguba and Kundelungu Groups siliciclastic rocks have elemental abundances and ratios suggestive of a relatively felsic TTG source, although slightly more mafic compositions occur in the Nguba Group and the overlying “Petit Conglomérat” Formation at the base of the Kundelungu Group. Modal compositions of the Nguba Group rocks indicate a basement uplift provenance, and geochemical parameters indicate the source of both the Nguba and Kundelungu Groups had an active continental margin character. Source area weathering was moderate in the Nguba Group. Low Chemical Index of Alteration (CIA) and Plagioclase Index of Alteration (PIA) indices and relatively uniform chemical compositions of the “Grand Conglomérat” and the “Petit Conglomérat” Formations lying respectively at the bases of the Nguba and Kundelungu Groups are compatible with deposition in a cool or frigid climate, and support their presumed petrographic based glaciogenic origin. High CIA and PIA indices in Upper Kalule rocks in the middle part of the Kundelungu Group point to the intensification of source weathering, possibly under tropical to subtropical climate under steady state conditions. Geochemical similarities between the Nguba Group and the “Petit Conglomérat” are compatible with a change from an extensional setting to compression, with derivation of the “Petit Conglomérat” by reworking of the underlying units during basin inversion. Change in provenance signatures and weathering indices in the Upper Kalule Formation may reflect reduced tectonism and resumption of supply of more weathered extrabasinal detritus, similar to that which fed the basal Roan Group. Overall the data suggest derivation mainly from pre-Katangan Proterozoic sources with continental arc characteristics. The adjacent Paleoproterozoic Ubendian Belt, particularly the Bangweulu block calcalkaline plutonic and volcanic province, is a suitable candidate as the source for the Nguba and Kundelungu Group sedimentary rocks. However, Mesoproterozoic and Archaean terrains have also contributed a minor component to the basin.

Tropical chemical weathering of hillslope deposits and bedrock source in the Aburrá Valley, northern Colombian Andes

Tropical chemical weathering produces extensive lateritization and formation of deep weathering profiles. Both processes are fundamental to landscape evolution and slope instability. The Aburrá Valley of the northern Colombian Andes is characterized by tropical conditions. The valley slopes are mostly covered by hillslope deposits originating from four basement rock suites which comprise contrasting granitoid, volcanic–sedimentary, ophiolitic, and metamorphic sources, respectively. Tropical chemical weathering of the Aburrá hillslope deposits and their respective bedrock were examined using X-ray fluorescence and X-ray diffraction analysis, to document and quantify their chemical weathering profiles, compositions, and mineralogical properties. The Chemical Index of Alteration (CIA), loss on ignition (LOI), and the Mobiles index (Imob) were used to quantify the degree of weathering of hillslope deposits and bedrock source. Weathering trends were analyzed using A–CN–K and A–CNK–FM diagrams. The material mantling the slopes in the Aburrá Valley records an intense weathering history. Chemical weathering is characterized by increased development of clay minerals (kaolinite, halloysite) and iron and aluminum sesquioxides. Lateritization characterizes the final stage of the weathering profiles. Concentrations of CaO, Na 2O, K 2O decrease markedly in the weathering products compared to the fresh bedrock source, whereas concentrations of Al 2O 3, Fe 2O 3, and MgO increase significantly. CIA ratios of matrix slope deposits derived from all four sources near 100, whereas those of boulder slope deposits and saprolites are lower, but exceed source rock values. Different A–CN–K weathering paths are evident for each lithotype, validating the correlation established between the hillslope deposits and their various parents. Chemical weathering indices in some samples are strongly influenced by the presence of sesquioxides, as reflected by high LOI, anomalously low CIA, and varying enrichment trends on the A–CNK–FM diagrams. Consequently, different chemical indices based on different criteria need to be combined to obtain best results, as illustrated here by the combination of LOI, CIA, and Imob. The overall results suggest that tropical conditions have dominated for a long time in the northern Colombian Andes, leading to uniformly high weathering indices in matrix slope deposits irrespective of parent lithotype. Prolonged warm and humid conditions could thus be responsible for the weathering and remobilization of extensive old hillslope deposits during the Quaternary. However, in addition to the influence of climatic factors, tectonism has also undoubtedly influenced slope evolution in the Aburrá Valley.

Geochemical characteristics of the metapelites from the Xingxingxia group in the Eastern Segment of the Central Tianshan: Implications for the provenance and paleoweathering

Metapelites from the Mesoproterozoic Xingxingxia group in the Eastern Segment of the Central Tianshan, Northwestern China, were analyzed for major and trace elements, including rare earth elements. Compared with post-Archean shales, the metapelites are enriched in Sr, Hf and Zr. The other elements are similar to the compositions of the Phanerozoic North American Shale Composite and the Post-Archean Australian average Shale (PAAS). The characteristics of Al 2 O 3 /TiO 2 , Cr/Zr, Cr/Th, Th/Sc and high Zr concentration suggest that these pelites could be derived from a significant proportion of felsic and a few proportion of participation of mafic materials in nature. These rocks display highly fractionated REE patterns, and average (La/Yb) n = 18.6. Several samples have slightly negative Eu anomalies (Eu/Eu* = 0.7―0.84) in comparison with PAAS (Eu/Eu* = 0.65), indicating contribution from young immature materials. The low K 2 O/Al 2 O 3 values of these metapelites suggest that their source materials could contain minimal alkali feldspar. In the A-CN-K triangular diagram, the sample data array intersects the feldspar join, indicating that the source had a plagioclase: K-feldspar ratio of approximately 5:1, representing weathered products from components that could approximately correspond to granodiorites and tonalites. The low Chemical Index of Alteration (CIA) and high Index of Compositional Variability (ICV) values suggest low degrees of weathering of the source and low compositional maturity of the sediments, respectively. The geochemical characteristics of the metapelites from the Xingxingxia group also demonstrate that these sediments were deposited in a back-arc background within a continental-arc system.

Variability in Geochemical Provenance and Weathering History of Sirius Group Strata, Transantarctic Mountains: Implications for Antarctic Glacial History

ABSTRACT The major-element and trace-element geochemistry of 57 diamicts and muds of the Sirius Group is evaluated to determine the provenance and stratigraphic relations of geographically widespread outcrops in the Transantarctic Mountains. The origin and age of the Sirius Group are heavily debated because of the presence of marine diatoms inferred to indicate a reduced East Antarctic ice sheet as recent as the Pliocene. A key question is whether glaciers transported sediments and the enclosed diatoms from the East Antarctic interior to the Transantarctic Mountains. Major-element and trace-element ratios indicate that the greater part of the Sirius Group sediments have an East Antarctic provenance and that a few high-elevation deposits are locally derived. The degree of chemical weathering of the materials in the Sirius Group, expressed as the chemical index of alteration (CIA), has a wide range (41-70). On the basis of geochemical provenance, weathering intensity, and morphostratigraphic position, the Sirius Group can be subdivided into three subgroups, which probably represent multiple glacial phases: (1) sediments with a local provenance from high-elevation outcrops on spurs and mountain summits, > 2000 m, which contain abundant weathered materials; (2) sediments in high-elevated outcrops, > 2000 m, with intermediate CIAs, originating from an East Antarctic Ice Sheet, which was overriding parts of the Transantarctic Mountains and; (3) sediments recovered within, or on the margins of, the present glacial troughs at 1500-1800 m, with low CIAs and an East Antarctic provenance. Comparison with the weathering history inferred from drilling records in the nearby Victoria Land Basin reveals that glaciation in the Transantarctic Mountains probably commenced with small ice caps and alpine glaciers prior to the Eocene-Oligocene transition, and from the Oligocene onward continued with multiple phases of continental glaciation. These results and interpretations contribute considerably to the understanding of the landscape evolution of the Transantarctic Mountains, the Sirius Group debate, and the history of East Antarctic glaciation.

Geochemistry of the Floodplain Sediments of the Kaveri River, Southern India

ABSTRACT The floodplain sediments of the Kaveri River, southern India, derived from Archean gneissic and charnockitic source regions, show interbedding of silty (4-4.7 ) and sandy units (1.4-3.7 ). The geochemistry of silty beds is remarkably uniform at a given location and over a lateral distance of nearly 250 km; the sandy beds have more variable chemical compositions, yet are comparable to those of silty beds except for the diluting effect of quartz. Silty sediments retain the geochemical signature of prominently exposed source rocks for almost all elements and provide evidence of tectonic instability in the source region. The floodplain sediments contain all size grades (sand, silt, and clay), which may have resulted in minimizing the biases imposed on suspended and bedload sediments due to sorting. The low Chemical Index of Alteration (CIA), the dominance of unweathered primary minerals, and the minor proportion of smectitic clay all suggest that the region has been subjected to little chemical weathering. This is possible if the region has undergone recent uplift, exposing fresh Archean rock to surface denudation. The formation of fertile farmland along the Kaveri River course and its delta is related to these recent geological processes.

Geochemical indications of provenance change as recorded in Miocene shales: opening of the Japan Sea, San'in region, southwest Japan

Shales from the San'in Region, Southwest Japan, exhibit geochemical patterns that record provenance change during the opening of the Japan Sea. Four formations, the Koura, Josoji, Ushikiri and Furue, are described. The basal Koura Formation is non-marine, while those overlying are marine. The Koura Formation shows variation from basic to silicic composition, due to fluvial transportation of source rocks with lower CIA (chemical index of alteration) than those of the Josoji Formation. The source material of the Koura Formation is mainly felsic (possibly plutonic) rocks, as shown by Cr Zr - Y Ni systematics, and similar to Ebi granitic rocks of the Hida continental massif in SiO 2 TiO 2 - Al 2O 3 TiO 2 ratios. The Josoji Formation is enriched in SiO 2 from high biogenic productivity. Ti Zr and Cr Zr - Y Ni ratios of the Josoji Formation suggest that basalt to andesite were significant lithologies in the source region. Provenance change from the Koura to the Josoji Formations was possibly related to the transgression caused by the first opening of the Japan Sea, where sediments were derived from deeply weathered material in the wider source areas. The Ushikiri Formation shows mainly intermediate composition, and is characterized by relatively higher Ca and Sr. This is indicative of uplift of the Paleogene igneous rocks in the source areas, with intermediate volcanic activity, probably related to the second opening event. The Furue Formation has similar composition to the Ushikiri Formation, but with comparatively higher CIA, suggesting another episode of deeper source area weathering, and well mixed source material. The results are consistent with published geological interpretations of the area, and show that the geochemistry of shales from active margins is a useful tool for examining the development of such margins. Locally derived shales retain source composition variability and signatures, while those deposited in deeper basins show more consistent chemistry closer to PAAS composition.

Major element geochemistry and paleoclimatology of the Permo-Carboniferous glacigene Dwyka Formation and postglacial mudrocks in southern Africa

A Chemical Index of Alteration (CIA) is used to quantify the degree of weathering of fine-grained materials in the glacigenic Permo-Carboniferous Dwyka Formation and succeeding stratigraphic units of southern Africa. Diamictites in the Dwyka Formation have low CIA values compatible with their glacigenic origin. Interbedded mudrocks show a wide scatter of values, suggesting that at least some of them were deposited during periods when ice had retreated from the source areas. Samples from the Prince Albert shale, which immediately overlies the Dwyka Formation, show high CIA values, suggesting a rapid transition to a much more intensive weathering regime (warmer climatic conditions?). Samples from the succeeding Whitehill shale have a low average value, so that weathering was once more inhibited, perhaps by a temporary return to a colder climate. The highest stratigraphic units sampled, the “upper shales”, have an average CIA value comparable to that of “average” shales, and suggesting “normal” weathering conditions. Major element geochemical data can supplement sedimentological and paleontological information and provide a valuable contribution to the detailed paleoclimatic interpretation of such glacigenic successions.

Samples and Principles

Major, trace element concentrations and Nd, Sr isotope ratios were measured in micro-drilled samples of a 2.37 Ga-old, hand-specimen sized spheroidally weathered diabase from southern India. A sample of the un-weathered diabase dike was also analyzed. X-ray micro-CT imaging of the weathered sample shows three dominant mineral phases which are plagioclase, pyroxene, and a Fe-bearing phase (possibly hematite and/or ilmenite). This imaging documents the pervasive nature of two generations of ribbon-like, cross-cutting fractures. The older fracture is sealed while the more recent fracture is open without any in-filling. The values of the Chemical Index of Alteration (CIA) of the samples show a wide range but are less than 50. Despite being a relatively less weathered rock, we observe that concentrations of major, minor and trace elements vary significantly with the percentage relative standard deviation (%RSD) for the elements ranging from 10.2–41.8. The CIA of the samples do not show any trend with the position of the sample in the hand-specimen. Barring Ca and Li, whose concentrations decrease from the core to the rim of the sample, there is no significant spatial trend in the concentrations of the elements.Concentrations of Na, Al, and Sr increase with increasing CIA values while concentrations of Mg, Fe, and Sc decrease with increasing CIA. The strong positive correlations of Na and Al, as well as Na and Sr indicates preferential weathering of plagioclase in the diabase. Na/Ca increases while Mg/Al, Mg/Na, Mg/Ca, Fe/Al and Sc/Sr decrease with increasing CIA values and the un-weathered rock plots in the middle of these trends. Such variations are explained in terms of differential weathering of plagioclase (in samples with lower CIA than the un-weathered rock, W1-type) and pyroxene (in samples with higher CIA than the un-weathered rock, W2-type) which have varying resistance to weathering. At the hand-specimen scale, the variability in the weathering indices like CIA are controlled by differential weathering of minerals and might not accurately reflect the intensity of weathering. Chondrite-normalized La/Sm and Gd/Lu co-vary with CIA values indicating mobility of the REEs during spheroidal weathering even at the hand-specimen scale. The Eu anomaly also increases with increasing CIA values which is explained by differential weathering of pyroxene and plagioclase. We observe large percentage deviations of the Nb-normalized concentrations of elements from the un-weathered rock in specific samples but no spatial trend is observed. Overall, the variations in element concentrations can be explained by varying fluid mobility of the elements, selective weathering of the minerals in the diabase, and ambient environmental conditions.Considerable Nd and Sr isotopic variability is observed at the hand-specimen scale and is explained in terms of weathering-related fractionation of parent/daughter ratios. This elemental fractionation must have happened long time ago to allow for radiogenic decay of the long-lived isotopes of 87Rb and 147Sm. The spread (%RSD) in the initial Sr and Nd isotope compositions of the weathered samples reach a minimum value around 1.2–1.3 Ga which we interpret as the timing of the peak weathering event which led to fractionation of the parent/daughter ratios. For Nd isotopes, the average εNd (1.2 Ga) of the weathered samples coincides with the εNd (1.2 Ga) of the un-weathered rock. The timing of the weathering event coincides with the timing of the breakup of the Columbia supercontinent and follows wide-spread alkaline volcanism in the Indian subcontinent. This is the first such attempt to determine the timing of a weathering event in rocks using long-lived radioactive isotopes.